Areas in the form of complex industrial plants normally have means for displaying information on the operation and on the monitoring of the plant. The monitoring comprises, on the one hand, the checking of process parameters and, on the other hand, the detection of accidents, which may develop, for example, because of leaky line systems. Prior-art monitoring systems for industrial plants are devices for measuring gas concentrations, which are arranged stationarily as stationary measuring heads, or they are mobile measuring systems, which are carried by persons. Such a device for measuring gas concentrations has become known from DE 198 58 022 C2. A satellite-supported positioning system GPS (global positioning system) is used to determine the position of the stationary and mobile measuring heads. However, these positioning systems cannot be used in closed spaces, and it is not possible to determine the direction of a movement because of the limited resolution.
Hazardous working areas are secured in industrial plants such that only certain persons can enter them. It is known from DE 33 21 790 A1 that an active or passive transponder is associated with individual pieces of clothing of the persons to be protected. A code, which contains a personal code, can be assigned to this transponder, so that only authorized persons, who carry certain items of equipment provided with transponders, can enter the hazardous areas. The prior-art personal monitoring system comprises no position recognition that would enable a head of operations sitting in a monitoring center to determine the instantaneous position of the employees.